/Engine/Source/Runtime/GeometryCore/Public/Util/RefCountVector.h
For example, we could rebuild ref counts for vertices by iterating over all triangles via the Iterate callable, and in the process any vertex we encounter for the first time gets a ref count of 1 via the AllocateRefCount callable, and any vertex we encounter repeatedly has its ref count incremented by one via the IncrementRefCount callable.
template<typename IterateFunc, typename AllocateRefCountFunc, typename IncrementRefCountFunc> void Rebuild ( unsigned int Num, IterateFunc && Iterate, AllocateRefCountFunc && AllocateRefCount, IncrementRefCountFunc && IncrementRefCount )